Activation from computer:
1. Log in to your account at http://play.google.com/redeem or create new account.
2. Input code of the card and tap Redeem

Activation from Android device:
1. Open Play Market
2. The Play Store menu button is on the upper left corner.
3. Tap on Redeem.
4. type in your Google Play gift card code (the one seen here is obviously not real), and hit Redeem